1. Field of the Invention
This invention relates to methods and apparatus concerned with the recording or erasure of images on thermoplastic material.
2. Description of the Prior Art
In the recording and erasing of deformation images on a thermoplastic photoconductive recording material, the material by charging by means of a corona device and subsequent exposure, carries a charge image which is thermally developed by heating up to the softening range of the recording material and which can be erased by renewed, longer heating at the same temperature or by heating at a higher temperature.
Such processes are suitable inter alia for recording and erasing holograms, for which in addition to silver halide films, photo-lacquered layers or manganese/bismuth layers, thermoplastic photoconductive layers in particular are used.
Such recording materials contain either a photoconductor dispersed in a thermoplastic or a photoconductive layer which is provided with a thermoplastic covering layer. Known photoconductors are poly-n-vinylcarbazole with an addition of trinitrofluorenone as well as copper phthalocyanin or substituted pyrenes. Customary thermoplastics are Staybelite Ester 10.RTM., a hydrogenated rosin ester, polystyrenes or polyacrylates. Such recording materials are electrostatically charged, exposed with a light-shadow pattern, possibly in the form of a hologram, if necessary charged again and developed by applying heat such as radiation or joulean heat. The softened thermoplastic layer is deformed in accordance with the charge image to form a deformation image which can be made visible by a schlieren technique. In the case of holographic recording a phase hologram is obtained. By renewed heating the deformation image or the phase hologram can be erased again. It is also possible for charge images to be produced on thermoplastic layers by charge transfer or directly, for example, by electron beam recording.
There is a great interest in recording and erasure of deformation images in the field of holography. The following standard technique is used for this: On a glass plate, as carrier plate for the photo-thermoplastic material, conductive areas of the magnitude of the holograms to be prepared are produced from transparent conductive layers such as tin oxide of approximately 50 ohm/square, over which joulean heat is supplied. Opposite sides of the conductive areas are strengthened by electrodes consisting of, for example, gold and are provided with lead wires. The necessary quantity of heat is produced by applying a voltage for a predetermined period.
The ranges of tolerance of the heat energy to be supplied to the thermoplastic layer for thermal development are only a few percent. If during the developing time the thermoplastic layer does not soften to the extent of being deformable by the electrostatic forces originating from the charge image then the layer surface remains smooth. If the thermoplastic layer is only slightly softer than is necessary then as a result of increased dark conductivity the electrostatic charges leak away so quickly that once the electrostatic forces of the image are no longer present the mechanical surface tension flattens the deformation image. In order to obtain good relief images the necessary quantity of heat must accordingly be dosed in such a manner that the narrow temperature range for the deformation of the thermoplastic layer is reached and maintained. In the case of constant voltage and time values, which is tantamount to a quantity of heat of constant magnitude, the system must therefore always be in the same initial thermal state. In practical operations it has therefore been necessary for any subsequent recording process to wait until the system has cooled again, for example to room temperature. Variations in the room temperature always require a re-adjustment of the heating device for the thermal development. In accordance with experience the cooling times last for some minutes, which has been found to be unsatisfactorily long for quick, and under certain circumstances irregular recording sequences, in particular the case of cyclic recordings with alternate recording and erasing operations. Although measures for accelerating the cooling, such as fans, can shorten the cooling time to some extent they require, in addition, periodic operation unless they are specially controlled.